Mutation of the five conserved histidines in the endothelial nitric-oxide synthase hemoprotein domain. No evidence for a non-heme metal requirement for catalysis

A Pilot Optical Coherence Tomography Angiography Study on Superficial and Deep Capillary Plexus Foveal Avascular Zone in Patients With Beta-Thalassemia Major

Purpose

To investigate foveal avascular zone (FAZ) changes in the superficial (SCP) and deep (DCP) capillary plexuses in beta-thalassemia major (BTM) patients, as shown in optical coherence tomography angiography.

Methods

Nonrandomized, comparative case series of 54 eyes of 27 BTM patients and 46 eyes of 23 healthy controls, utilizing an automated FAZ detection algorithm. Measurements included FAZ area and FAZ shape descriptors (convexity, circularity, and contour temperature). Results were compared between the two groups, and correlated to iron load and chelation therapy parameters.

Results

SCP and DCP FAZ area were not significantly different between the control and BTM groups (P = 0.778 and P = 0.408, respectively). The same was true regarding SCP FAZ convexity (P = 0.946), circularity (P = 0.838), and contour temperature (P = 0.907). In contrast, a statistically significant difference was detected between controls and BTM group regarding DCP FAZ convexity (P = 0.013), circularity (P = 0.010), and contour temperature (P = 0.014). Desferrioxamine dosage was strongly correlated to the DCP area (r = 0.650, P = 0.05) and liver magnetic resonance imaging/T2-star to DCP circularity (r = -0.492, P = 0.038). Correlations were also revealed between urine Fe excretion and DCP convexity (r = 0.531, P = 0.019), circularity (r = 0.661, P = 0.002), and contour temperature (r = -0.591, P = 0.008).

Conclusions

Retinal capillary plexuses and especially DCP seem to present unique morphologic changes in BTM patients, not in the FAZ area, but in specific shape descriptors, indicating minor but detectable FAZ changes. These changes correlate well with iron load and chelation therapy parameters. Their clinical importance and pathophysiologic implications remain to be elucidated through further studies.

Binding of PDZ domains to the carboxy terminus of inducible nitric oxide synthase boosts electron transfer and NO synthesis

iNOS lacks any phosphorylatable residue at its C-terminus despite displaying a 25-residue extension known to block electron transfer and activity. We report that C-terminal deletions of iNOS increased the cytochrome c reduction rate. Moreover, the interaction of the iNOS C-terminus with the PDZ domains of EBP50 or CAP70 resulted not only in augmented reductase activity and greater NO synthesis but also anticipated the formation of the air-stable semiquinone generated after NADPH addition. Hence, the C-terminus of iNOS regulates the activity of the enzyme, albeit, unlike nNOS and eNOS, displacement of the autoinhibitory element occurs upon binding to proteins with PDZ domains.

Protein kinase D activity controls endothelial nitric oxide synthesis

Vascular endothelial growth factor (VEGF) regulates key functions of the endothelium, such as angiogenesis or vessel repair in processes involving endothelial nitric oxide synthase (eNOS) activation. One of the effector kinases that become activated in endothelial cells upon VEGF treatment is protein kinase D (PKD). Here, we show that PKD phosphorylates eNOS, leading to its activation and a concomitant increase in NO synthesis. Using mass spectrometry, we show that the purified active kinase specifically phosphorylates recombinant eNOS on Ser1179. Treatment of endothelial cells with VEGF or phorbol 12,13-dibutyrate (PDBu) activates PKD and increases eNOS Ser1179 phosphorylation. In addition, pharmacological inhibition of PKD and gene silencing of both PKD1 and PKD2 abrogate VEGF signaling, resulting in a clear diminished migration of endothelial cells in a wound healing assay. Finally, inhibition of PKD in mice results in an almost complete disappearance of the VEGF-induced vasodilatation, as monitored through determination of the diameter of the carotid artery. Hence, our data indicate that PKD is a new regulatory kinase of eNOS in endothelial cells whose activity orchestrates mammalian vascular tone.

Protein kinase D interacts with neuronal nitric oxide synthase and phosphorylates the activatory residue serine 1412

Neuronal Nitric Oxide Synthase (nNOS) is the biosynthetic enzyme responsible for nitric oxide (·NO) production in muscles and in the nervous system. This constitutive enzyme, unlike its endothelial and inducible counterparts, presents an N-terminal PDZ domain known to display a preference for PDZ-binding motifs bearing acidic residues at -2 position. In a previous work, we discovered that the C-terminal end of two members of protein kinase D family (PKD1 and PKD2) constitutes a PDZ-ligand. PKD1 has been shown to regulate multiple cellular processes and, when activated, becomes autophosphorylated at Ser⁹¹⁶, a residue located at -2 position of its PDZ-binding motif. Since nNOS and PKD are spatially enriched in postsynaptic densities and dendrites, the main objective of our study was to determine whether PKD1 activation could result in a direct interaction with nNOS through their respective PDZ-ligand and PDZ domain, and to analyze the functional consequences of this interaction. Herein we demonstrate that PKD1 associates with nNOS in neurons and in transfected cells, and that kinase activation enhances PKD1-nNOS co-immunoprecipitation and subcellular colocalization. However, transfection of mammalian cells with PKD1 mutants and yeast two hybrid assays showed that the association of these two enzymes does not depend on PKD1 PDZ-ligand but its pleckstrin homology domain. Furthermore, this domain was able to pull-down nNOS from brain extracts and bind to purified nNOS, indicating that it mediates a direct PKD1-nNOS interaction. In addition, using mass spectrometry we demonstrate that PKD1 specifically phosphorylates nNOS in the activatory residue Ser¹⁴¹², and that this phosphorylation increases nNOS activity and ·NO production in living cells. In conclusion, these novel findings reveal a crucial role of PKD1 in the regulation of nNOS activation and synthesis of ·NO, a mediator involved in physiological neuronal signaling or neurotoxicity under pathological conditions such as ischemic stroke or neurodegeneration.

Covalent attachment of heme to the protein moiety in an insect E75 nitric oxide sensor

We have recombinantly expressed and purified the ligand binding domains (LBDs) of four insect nuclear receptors of the E75 family. The Drosophila melanogaster and Bombyx mori nuclear receptors were purified as ferric hemoproteins with Soret maxima at 424 nm, whereas their ferrous forms had a Soret maximum at 425 nm that responds to (•)NO and CO binding. In contrast, the purified LBD of Oncopeltus fasciatus displayed a Soret maximum at 415 nm for the ferric protein that shifted to 425 nm in its ferrous state. Binding of (•)NO to the heme moiety of the D. melanogaster and B. mori E75 LBD resulted in the appearance of a peak at 385 nm, whereas this peak appeared at 416 nm in the case of the O. fasciatus hemoprotein, resembling the behavior displayed by its human homologue, Rev-erbβ. High-performance liquid chromatography analysis revealed that, unlike the D. melanogaster and B. mori counterparts, the heme group of O. fasciatus is covalently attached to the protein through the side chains of two amino acids. The high degree of sequence homology with O. fasciatus E75 led us to clone and express the LBD of Blattella germanica, which established that its spectral properties closely resemble those of O. fasciatus and that it also has the heme group covalently bound to the protein. Hence, (•)NO/CO regulation of the transcriptional activity of these nuclear receptors might be differently controlled among various insect species. In addition, covalent heme binding provides strong evidence that at least some of these nuclear receptors function as diatomic gas sensors rather than heme sensors. Finally, our findings expand the classes of hemoproteins in which the heme group is normally covalently attached to the polypeptide chain.

Circulating CD133(+)VEGFR2 (+) and CD34 (+)VEGFR2 (+) cells and arterial function in patients with beta-thalassaemia major

Arterial dysfunction has been documented in patients with beta-thalassaemia major. This study aimed to determine the quantity and proliferative capacity of circulating CD133(+)VEGFR2(+) and CD34(+)VEGFR2(+) cells in patients with beta-thalassaemia major and those after haematopoietic stem cell transplantation (HSCT), and their relationships with arterial function. Brachial arterial flow-mediated dilation (FMD), carotid arterial stiffness, the quantity of these circulating cells and their number of colony-forming units (CFUs) were determined in 17 transfusion-dependent thalassaemia patients, 14 patients after HSCT and 11 controls. Compared with controls, both patient groups had significantly lower FMD and greater arterial stiffness. Despite having increased CD133(+)VEGFR2(+) and CD34(+)VEGFR2(+) cells, transfusion-dependent patients had significantly reduced CFUs compared with controls (p = 0.002). There was a trend of increasing CFUs across the three groups with decreasing iron load (p = 0.011). The CFUs correlated with brachial FMD (p = 0.029) and arterial stiffness (p = 0.02), but not with serum ferritin level. Multiple linear regression showed that CFU was a significant determinant of FMD (p = 0.043) and arterial stiffness (p = 0.02) after adjustment of age, sex, body mass index, blood pressure and serum ferritin level. In conclusion, arterial dysfunction found in patients with beta-thalassaemia major before and after HSCT may be related to impaired proliferation of CD133(+)VEGFR2(+) and CD34(+)VEGFR2(+) cells.

Elastic Properties of the Ascending Aorta in Patients with ?-Thalassemia Major

BACKGROUND

Beta-thalassemia major (beta-TM) is a congenital hemolytic disorder characterized by impaired left ventricular and endothelial function. However, elastic properties of the aorta have not been sufficiently investigated in patients with beta-TM. We investigated whether beta-TM is related to impaired ascending aortic elastic properties.

METHODS

We studied 36 patients with beta-TM (age: 15.8 +/- 2.6 years) and 30 age- and sex-matched control subjects by echocardiography. Aortic elastic indexes, aortic strain (%), distensibility (cm(2) dyn(-1) 10(-3)), and stiffness index were calculated from the echocardiographically derived thoracic aortic diameters (mm/m(2)), and the measurement of pulse pressure obtained by cuff sphygmomanometry.

RESULTS

Patients versus control subjects had increased aortic diameters (P < 0.001), lower mean aortic strain (9 +/- 3.6 vs. 14.9 +/- 3.2, P < 0.001) and distensibility (0.6 +/- 0.36 vs. 0.8 +/- 0.2, P < 0.012), and higher mean stiffness index (5.3 +/- 2.4 vs. 2.8 +/- 0.6, P < 0.001). Aortic elastic indexes were significantly associated with ferritin level, while stiffness index was significantly related to platelet count.

CONCLUSION

Elastic properties of ascending aorta are impaired in patients with beta-TM. Impaired functions of aorta may lead to deterioration of left ventricular function.

Intravenous iron increases labile serum iron but does not impair forearm blood flow reactivity in dialysis patients

BACKGROUND

There are concerns about adverse vascular effects of intravenous iron by inducing oxidative stress. We therefore examined the effect of a single high dose of intravenous iron on endothelial function and biochemical markers of iron homeostasis.

METHODS

In a randomized, placebo-controlled, double-blind, parallel-group study, forearm blood flow (FBF) was assessed by strain-gauge plethysmography in 38 peritoneal dialysis patients before and after a single intravenous infusion of 300 mg iron sucrose.

RESULTS

Iron infusion increased total (Delta 601 microg/100 mL, CI 507, 696) and non-transferrin-bound iron (Delta 237.2 micromol/L, CI 173.6, 300.8) approximately 10-fold, as well as redox-active iron nearly five-fold (Delta 0.76 micromol/L, CI 0.54, 0.98). After iron infusion basal FBF was 59% higher than after placebo. FBF response to acetylcholine before and after iron infusion was 263 +/- 32% and 310 +/- 33%, corresponding to 304 +/- 43% and 373 +/- 29% in the placebo group, respectively. Before and after iron or placebo infusion, glyceryl-trinitrate increased resting FBF to 232 +/- 22% and 258 +/- 21% in the iron group, and to 234 +/- 18% and 270 +/- 30% in the placebo group. L-N-monomethyl-arginine decreased FBF to 70 +/- 4% and 72 +/- 3% before and after iron, and to 74 +/- 4% and 73 +/- 4% before and after placebo infusions, respectively. Despite higher basal FBF after iron infusion, absolute and relative FBF changes in response to vasoactive substances were not significantly different between iron and placebo groups.

CONCLUSION

Our data suggest that 300 mg intravenous iron sucrose has a vasodilatory effect, but does not impair vascular reactivity in dialysis patients, despite a significant increase in non-transferrin-bound and redox-active iron.

Mutating His29, His125, His133 or His158 abolishes glycosylphosphatidylinositol-specific phospholipase D catalytic activity

Glycosylphosphatidylinositol (GPI)-specific phospholipase D (GPI-PLD) specifically cleaves GPIs. This phospholipase D is a secreted protein consisting of two domains: an N-terminal catalytic domain and a predicted C-terminal b-propeller. Although the biochemical properties of GPI-PLD have been extensively studied, its catalytic site has not been identified. We hypothesized that a histidine residue(s) may play a critical role in the catalytic activity of GPI-PLD, based on the observations that (i) Zn2+, which utilizes histidine residues for binding, is required for GPI-PLD catalytic activity, (ii) a phosphohistidine intermediate is involved in phospholipase D hydrolysis of phosphatidylcholine, (iii) computer modelling suggests a catalytic site containing histidine residues, and (iv) our observation that diethyl pyrocarbonate, which modifies histidine residues, inhibits GPI-PLD catalytic activity. Individual mutation of the ten histidine residues to asparagine in the catalytic domain of murine GPI-PLD resulted in three general phenotypes: not secreted or retained (His56 or His88), secreted with catalytic activity (His34, His81, His98 or His219) and secreted without catalytic activity (His29, His125, His133 or His158). Changing His133 but not His29, His125 or His158 to Cys resulted in a mutant that retained catalytic activity, suggesting that at least His133 is involved in Zn2+ binding. His133 and His158 also retained the biochemical properties of wild-type GPI-PLD including trypsin cleavage pattern and phosphorylation by protein kinase A. Hence, His29, His125, His133 and His158 are required for GPI-PLD catalytic activity.

Direct interaction between the reductase domain of endothelial nitric oxide synthase and the ryanodine receptor

We have performed the recombinant expression and purification of the reductase domain of endothelial nitric oxide synthase (eNOS) and used it as a bait in search for interacting proteins present in endothelial cells. Using mass spectrometry of the bound proteins run in a PAGE-SDS gel, we were able to identify the ryanodine receptor (RyR) as a novel eNOS-binding partner. This interaction was confirmed through immunoprecipitation of both RyR and eNOS from endothelial cells and cardiac myocytes. Immunofluorescence data indicated that a subpopulation of eNOS associates with RyR in perinuclear regions of the cell, where eNOS might be responsible for the known nitrosylation of RyR.

Biology and chemistry of the inhibition of nitric oxide synthases by pteridine-derivatives as therapeutic agents

Inhibitors of the family of nitric oxide synthases (NOS-I-III; EC 1.14.13.39) are of interest as pharmacological agents to modulate pathologically high nitric oxide (NO) levels in inflammation, sepsis, and stroke. In this article, we discuss the approach for targeting the unique (6R)-5,6,7,8-tetrahydro-L-biopterin (H4Bip) binding site of NOS by appropriate inhibitors. This binding site maximally increases enzyme activity and NO production from the substrate L-arginine upon cofactor binding. The first generation of H4Bip-based NOS inhibitors was based on 4-amino H4Bip derivatives in analogy to anti-folates such as methotrexate. In addition, we discuss the structure-activity relationship of a related series of 4-oxo-pteridine derivatives. Furthermore, molecular modeling studies provide an understanding of pterin antagonism on a structural level based on favorable and unfavorable interactions between protein binding site and ligands. These techniques include 3D-QSAR (CoMFA, CoMSIA) to understand ligand affinity and GRID/consensus principal component analysis (CPCA) to learn about selectivity requirements. Collectively these approaches, in combination with the presented SAR and structural data, provide useful information for the design of novel NOS inhibitors with increased isoform selectivity.

Induction of nitric oxide synthase-2 proceeds with the concomitant downregulation of the endogenous caveolin levels

Several cell types express inducible nitric oxide synthase (NOS2) in response to exogenous insults such as bacterial lipopolysaccharide (LPS) or proinflammatory cytokines. For instance, muscular cells treated with LPS and interferon gamma (IFN-gamma) respond by increasing the mRNA and protein levels of NOS2, and synthesize large amounts of nitric oxide. We show here that transcriptional induction of NOS2 in muscular cells proceeds with a concomitant decrease in the levels of caveolin-1, -2 and -3. Addition of *NO-releasing compounds to C2C12 muscle cells reveals that this downregulation of the caveolin (cav) levels is due to the presence of *NO itself in the case of caveolin-3 and to the action of the LPS/IFN-gamma in the case of cav-1 and cav-2. Likewise, muscle cells obtained from NOS2(-/-) knockout mice challenged with LPS/IFN-gamma could downregulate their levels of cav-1 but not of cav-3, unlike wild-type animals, in which both cav-1 and cav-3 levels diminished in the presence of the proinflammatory insult. Laser confocal immunofluorescence analysis proves that *NO exerts autocrine and paracrine actions, hence diminishing the cav-3 levels. When the induced NOS2 was purified using an affinity resin or immunoprecipitated from muscular tissues, it appears strongly bound not only to calmodulin but also to cav-1, and marginally to cav-2 and cav-3. When the cav levels where reduced using antisense oligonucleotides, an increase in the NOS2-derived.NO levels could be measured, demonstrating the inhibitory role of the three cav isoforms. Our results show that cells expressing NOS2 diminish their cav levels when the synthesis of *NO is required.

Distinct influence of N-terminal elements on neuronal nitric-oxide synthase structure and catalysis

Nitric oxide (NO) is a signal molecule produced in animals by three different NO synthases. Of these, only NOS I (neuronal nitric-oxide synthase; nNOS) is expressed as catalytically active N-terminally truncated forms that are missing either an N-terminal leader sequence required for protein-protein interactions or are missing the leader sequence plus three core structural motifs that in other NOS are required for dimer assembly and catalysis. To understand how the N-terminal elements impact nNOS structure-function, we generated, purified, and extensively characterized variants that were missing the N-terminal leader sequence (Delta296nNOS) or missing the leader sequence plus the three core motifs (Delta349nNOS). Eliminating the leader sequence had no impact on nNOS structure or catalysis. In contrast, additional removal of the core elements weakened but did not destroy the dimer interaction, slowed ferric heme reduction and reactivity of a hemedioxy intermediate, and caused a 10-fold poorer affinity toward substrate l-arginine. This created an nNOS variant with slower and less coupled NO synthesis that is predisposed to generate reactive oxygen species along with NO. Our findings help justify the existence of nNOS N-terminal splice variants and identify specific catalytic changes that create functional differences among them.

Arterial stiffness and endothelial function in patients with beta-thalassemia major

BACKGROUND

Increased iron store has been linked to risk of cardiovascular disease. Structural alterations of arteries in beta-thalassemia major patients and in vitro functional disturbance of vascular endothelial cells by thalassemic serum have been described. We sought to determine whether arterial stiffness and endothelial function are altered in vivo.

METHODS AND RESULTS

Thirty thalassemia patients (16 male) aged 22.2+/-7.4 years were recruited. Left ventricular (LV) mass and function were assessed echocardiographically. Carotid and brachioradial artery stiffness was assessed by stiffness index and pulse-wave velocity (PWV), respectively. Brachial artery endothelial function was assessed by vascular response to reactive hyperemia (flow-mediated dilation [FMD]) and sublingual glyceryl trinitrate. These indexes were compared with those of 30 age- and sex-matched controls. None of the patients had LV systolic or diastolic dysfunction. When compared with controls, patients had greater absolute (113.8+/-38.0 versus 109.0+/- 32.6 g, P=0.04) and indexed (82.4+/-17.5 versus 66.7+/-12.7 g/m(2), P<0.001) LV mass, carotid artery stiffness index (8.1+/-3.5 versus 5.5+/-1.6, P<0.001), and brachioradial PWV (8.9+/-2.4 versus 7.9+/-1.7 m/s, P= 0.03). Their FMD was impaired (3.5+/-3.3% versus 8.8+/-3.9%, P<0.001), whereas glyceryl trinitrate- mediated dilation was preserved (17.9+/-7.6% versus 16.3+/-6.1%, P=0.40). Both stiffness index and PWV correlated inversely with magnitude of FMD (r=-0.40, P=0.03; r=-0.41, P=0.03) and positively with indexed LV mass (r=0.50, P=0.005; r=0.40, P=0.027). Nonetheless, no significant correlation existed between ferritin level and carotid stiffness, PWV, or FMD.

CONCLUSIONS

Increased arterial stiffness, endothelial dysfunction, and LV hypertrophy occur in patients with beta-thalassemia major, which may result in reduction of mechanical efficiency of the heart.

Iron chelation improves endothelial function in patients with coronary artery disease

BACKGROUND

Some epidemiological studies have shown that increased iron stores are associated with increased cardiovascular events. Redox-active iron may contribute to lipid peroxidation, endothelial cell activation, and generation of reactive oxygen species (especially hydroxyl radical, via Fenton chemistry). Increased oxidative stress is associated with impaired action of endothelium-derived nitric oxide in patients with atherosclerosis.

METHODS AND RESULTS

To test the hypothesis that reducing vascular iron stores would reverse endothelial dysfunction, we examined the effects of the iron chelator deferoxamine (500 mg intra-arterially over 1 hour) on vasomotor function in forearm resistance vessels of patients with coronary artery disease by venous occlusion plethysmography. Patients with coronary artery disease had impaired endothelium-dependent vasodilation in response to methacholine compared with healthy control subjects (P<0.001). Deferoxamine infusion decreased serum iron levels (P<0.001). Deferoxamine improved the blood flow response to methacholine in patients with coronary artery disease (P<0.01 by 2-way repeated-measures ANOVA) but had no effect on the response to sodium nitroprusside. In normal volunteers, deferoxamine had no effect on the response to methacholine. The nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine abolished augmentation of the methacholine response associated with deferoxamine. The hydroxyl radical scavenger mannitol had no effect on the methacholine response.

CONCLUSIONS

Deferoxamine improved nitric oxide-mediated, endothelium-dependent vasodilation in patients with coronary artery disease. These results suggest that iron availability contributes to impaired nitric oxide action in atherosclerosis.

Reconstitution of pterin-free inducible nitric-oxide synthase

Inducible nitric-oxide synthase (NOS) was expressed and purified in the absence of 6(R)-tetrahydro-l-biopterin (H(4)B). Pterin-free NOS exhibits a Soret band (416-420 nm) characteristic of predominantly low spin heme and does not catalyze the formation of nitric oxide (. NO) (Rusche, K. M., Spiering, M. M., and Marletta, M. A. (1998) Biochemistry 37, 15503-15512). Reconstitution of pterin-free NOS with H(4)B was monitored by a shift in the Soret band to 396-400 nm, the recovery of.NO-forming activity, and the measurement of H(4)B bound to the enzyme. As assessed by these properties, H(4)B binding was not rapid and required the presence of a reduced thiol. Spectral changes and recovery of activity were incomplete in the absence of reduced thiol. Full reconstitution of holoenzyme activity and stoichiometric H(4)B binding was achieved in the presence of 5 mm glutathione (GSH). Preincubation with GSH before the addition of H(4)B decreased, whereas lower concentrations of GSH extended, the time required for reconstitution. Six protected cysteine residues in pterin-free NOS were identified by labeling of NOS with cysteine-directed reagents before and after reduction with GSH. Heme and metal content of pterin-free and H(4)B-reconstituted NOS were also measured and were found to be independent of H(4)B content. Additionally, pterin-free NOS was reconstituted with 6-methylpterin analogs, including redox-stable deazapterins. Reconstitution with the redox-stable pterin analogs was neither time- nor thiol-dependent. Apparent binding constants were determined for the 6-methyl- (50 microm) and 6-ethoxymethyl (200 microm) deazapterins. The redox-stable pterin analogs appear to bind to NOS in a different manner than H(4)B.

[Transition metals and nitric oxide production in human endothelial cells]

The bioavailability of endothelial nitric oxide (NO) is regulated by transition metals but their mechanisms of action on NO synthesis and degradation are not clearly understood. Using differential pulse amperometry and NO microelectrodes, local NO concentration was measured at the surface of cultured human umbilical vein endothelial cells (HUVECs) stimulated by histamine or thrombin in the presence of transition metal chelators. The agonist-activated NO release required both extracellular Ca2+ and transition metals. In the presence of 1 mM external Ca2+, a low concentration of EGTA (5 microM) inhibited by 40% the NO release from stimulated HUVECs. In the presence of extracellular L-arginine, the inhibitory effect of EGTA was even more marked and, in its absence, it was suppressed by adding exogenous superoxide dismutase. The decrease in NO release induced by the copper chelators, cuprizone and DETC, suggests that extracellular traces of Cu2+ could regulate NO availability.

Role of bound zinc in dimer stabilization but not enzyme activity of neuronal nitric-oxide synthase

Nitric-oxide synthases (NOS) are homodimeric proteins and can form an intersubunit Zn(4S) cluster. We have measured zinc bound to NOS purified from pig brain (0.6 mol/mol of NOS) and baculovirus-expressed rat neuronal NOS (nNOS) (0.49 +/- 0.13 mol/mol of NOS), by on-line gel-filtration/inductively coupled plasma mass spectrometry. Cobalt, manganese, molybdenum, nickel, and vanadium were all undetectable. Baculovirus-expressed nNOS also bound up to 2. 00 +/- 0.58 mol of copper/mol of NOS. Diethylenetriaminepentaacetic acid (DTPA) reduced the bound zinc to 0.28 +/- 0.07 and the copper to 0.97 +/- 0.24 mol/mol of NOS. Desalting of samples into thiol-free buffer did not affect the zinc content but completely eliminated the bound copper (</=0.02 mol/mol of NOS). Most (> or =75%) of the bound zinc was released from baculovirus-expressed rat nNOS by p-chloromercuriphenylsulfonic acid (PMPS). PMPS-treated nNOS was strongly (90 +/- 5%) inactivated. To isolate functional effects of zinc release from other effects of PMPS, PMPS-substituted thiols were unblocked by excess reduced thiol in the presence of DTPA, which hindered reincorporation of zinc. The resulting enzyme contained 0.12 +/- 0.05 mol of zinc but had a specific activity of 426 +/- 46 nmol of citrulline.mg(-1).min(-1), corresponding to 93 +/- 10% of non-PMPS-treated controls. PMPS also caused dissociation of nNOS dimers under native conditions, an effect that was blocked by the pteridine cofactor tetrahydrobiopterin (H(4)biopterin). H(4)biopterin did not affect zinc release. Even in the presence of H(4)biopterin, PMPS prevented conversion of NOS dimers to an SDS-resistant form. We conclude that zinc binding is a prerequisite for formation of SDS-resistant NOS dimers but is not essential for catalysis.